1. Field of the Invention
This invention relates to the field of ultrasonic analysis of bone tissue in humans, and more particularly to an improvement in the calibration and quality assurance of an ultrasonic bone analysis apparatus by using, for example, phantoms.
2. Description of the Related Art
The use of ultrasound in methods for detecting changes in bone characteristics is known. In particular, an ultrasound bone analysis apparatus has been used to analyze the properties of the heel bone or os calcis. The use of ultrasound is advantageous because it is non-invasive and is well-suited to repeated measurements or studies during medication since no ionizing radiation is used.
Precision and reliability of the ultrasonic bone analysis apparatus, as with other medical diagnostic instrumentation, are a matter of substantial importance. Therefore, the apparatus undergoes calibration and quality assurance regularly during its lifetime. Rather than using a human subject, the calibration and quality assurance is performed using a substitute medium that has specific ultrasonic properties. The calibration and quality assurance facilitate adjustment of the apparatus according to the specification of the instrument.
An ultrasonic bone analysis apparatus typically measures the rate of change of attenuation of ultrasound with frequency in the range of 200 to 600 kHz ("broadband ultrasound attenuation" or "BUA"), and also the speed of passage of acoustic waves ("speed of sound" or "SOS") through the bone. The BUA is a relative quantity calculated using a baseline signal as a reference of the transmitted signal entering the bone.
The baseline is typically acquired by measuring the signal after passage through a reference medium. Because the reference signal is used to assess the transmitted signal, the reference medium should minimally affect the ultrasonic signal.
Some existing ultrasonic bone analysis systems require immersing the foot of the patient in water. Several of these same "wet systems" use water as the reference medium. While water itself generally has a minimal distortive effect on the ultrasonic signal, its use necessitates preparation and cleanup, and is inconvenient for at least these reasons.
While some commercially available phantoms are suitable for monitoring temporal changes in scanner performance, the acoustic properties of these phantoms are typically significantly different from those of bones such as the os calcis. Therefore, these phantoms might not adequately mimic the human foot.
Heretofore, Clarke et al. proposed in "A Phantom for Quantitative Ultrasound of Trabecular Bone", 39 Phys. Med. Biol. 1677-87, to use a phantom as a substitute medium in a wet system. The proposed phantom consists of a rectangular block manufactured from a mixture of liquid epoxy and gelatine particles. While the proposed phantom does have acoustic properties similar to bone and may be adequate for experimental purposes, Clarke et al. admit that the proposed phantom has a number of unsolved practical problems such as durability.
A phantom manufactured from an epoxy and glass bead mixture has also been used with a wet system. However, the manufacture of this phantom is believed to be complex and to require substantial supervision and control.
The measurement of SOS depends on the ambient conditions. Measuring accurately and comparing SOS data can be difficult due to the wide range of possible conditions, and such difficulties can be aggravated by imprecise control and determination of the conditions of the measurement.
Various media have been used for testing SOS measurements. Pure water and saline solution of various sodium chloride concentrations have been employed. However, the SOS for each of these substances varies according to temperature, each substance having a positive temperature coefficient. Therefore, using one of these substances in the testing of the SOS measurements has the disadvantage that temperature is an additional variable.
While ethyl alcohol is known to have a negative temperature coefficient of sound propagation, mixing ethyl alcohol with water tends to cancel the positive temperature coefficient of sound propagation in the water. A mixture of 17% ethyl alcohol by weight has a substantially zero temperature coefficient of sound propagation over a large range of temperatures.